Why do we pay sales tax on tax?

When I buy cigarettes much of the price is tax, then I pay sales tax on the entire price of the pack....

I think the same thing happens with booze.

 

Dontcha just luv "sin taxes".
(I know, lets put a tax on tea. That'll learn em.)

 

'Cause the gubmint needs money, and what better way than to tax peoples' needs and wants?